The link between an ancient volcanic eruption and the world's most severe extinction event has been strengthened by new findings.
New analysis of mercury isotopes provides evidence that a quarter of a billion years ago, faraway places in the Southern Hemisphere were blanketed in debris from volcanic eruptions in Siberia.
The so-called Great Dying, also called the Permian-Triassic mass extinction event, occurred when most of life was wiped out in an ash-filled sky.
While it's clear this event ended in the loss of over 90% of marine species and over 70% of land vertebrates, our understanding of how Earth's greatest death event occurred remains a mystery.
By piecing together traces of chemicals trapped in rocks and marine sediments, geoscientists are reasonably certain that a series of volcanic eruptions unleashed dramatic changes in Earth's atmosphere and oceans that eventually suffocated animals.
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But an extinction event on the scale of the Great Dying also needs a pretty solid case before geoscientists can say for sure what caused it, and when it happened. They dug back in time to about 252 million years.
In previous research, zinc and nickel were used to link changes in ocean chemistry to massive volcanism and loss of marine life. But these elements are recycled at the Earth's surface, unlike mercury isotopes which offer a much more stable signal of volcanic activity.
Also, much of the research on these mass extinction events has focused on sites from the Northern Hemisphere, making it difficult to understand the impact of volcanism on the bottom of the Earth. This is significant because mounting evidence suggests that the Great Dying was not a single lethal event, but consisted of multiple extinction episodes that occurred in waves over a hundred thousand years.
So paleoclimatologist Jun Shen of China University of Geosciences and colleagues began detecting mercury isotopes in rock deposits at two Southern Hemisphere locations, namely the Karoo Basin in south-central Africa and the Sydney Basin on the east coast of Australia.
At the time of the Great Dying, the basins were united in one supercontinent called Pangaea, but are now about 10,000 kilometers apart and the Indian ocean. In it, the researchers found a nearly identical pattern: mercury isotopes peaked around the end of the Permian.
This evidence, from the farthest terrestrial site of the Siberian Traps to the present day, of giant lava flows formed by late epoch volcanoes, suggests that mercury exploded from volcanoes in the Northern Hemisphere and swept across the globe, researchers say.
"It turns out that volcanic emissions of mercury have a very specific isotopic composition of mercury that accumulates at the extinction horizon," explained study author and University of Connecticut geologist Tracy Frank, quoted from Science Alert.
"Knowing the age of these deposits, we can more definitely attribute the time of extinction to this major eruption in Siberia," he said.
Their work is in line with signals from sulfur isotopes that coincide with the Great Dying, and also builds on previous research showing mass extinctions began to occur on land up to 600,000 years before marine life was sucked out and largely disappeared.
"This shows that the event itself wasn't just one big, instantaneous blow," explains Christopher Fielding, another geologist at the University of Connecticut.
"It wasn't just one very bad event on Earth, and it gives new results because it shows volcanism is the root cause."
Researchers acknowledge that pinpointing the immediate cause of the Great Dying is not easy. A plume of ash from a volcanic eruption in southern China was also involved in the 'massacre', as well as the Siberian Traps.
Perhaps a more salient message to keep in mind is the fragility of life on a planet that is currently under the same climate change pressures, namely rising Earth temperatures and heat from greenhouse gases.